MOSCOW, Mar 25 (Reuters) - Russian prosecutors searched Amnesty International's Moscow offices on Monday in what activists say is a part of a state campaign to stifle dissent.

Since returning to the Kremlin in May, President Vladimir Putin has signed laws to tighten controls on non-governmental organisations, requiring those with foreign funding to register as "foreign agents", a term echoing Cold War era hostilities.

The Kremlin says the legislation is needed to prevent groups from spying for foreign governments, but Putin's critics see raids by state authorities ranging from tax officials to fire inspectors as harassment.

"This inspection was not planned," said Sergei Nikitin, Russia director of Amnesty International.

The London-based human rights advocacy group, one of several dozen NGOs to be searched in recent weeks, said officials from the Russian prosecutors' and tax offices declined to specify a reason for the search.

"The subject of the searches is NGOs' operations compliance with statutory goals and the Russian law," RIA news agency quoted the Russian Justice Ministry as saying.

Rights campaigners say the searches are probably aimed at gathering evidence of activities that would oblige them to register as "foreign agents" under the law.

The penalties for failing to comply include six months' suspension without a court order and, for individuals, up to three years in jail.

Russia's leading rights organisations, including the country's oldest rights group Memorial, election-monitoring body Golos and the Moscow Helsinki Group, said they would refuse to register in defiance of the law and would challenge it at the European Court of Human Rights.

Pavel Chikov, the head of Russian lawyers' association AGORA, said 41 NGOs had been searched so far, including Memorial.

"It is threatening ... when you see it as a national attack on thousands of organisations," Chikov said.

The Kremlin denies cracking down on critics but the searches have prompted criticism within Russia and from the West.

Putin's own advisory council on human rights has asked Prosecutor General Yuri Chaika to explain the wave of searches.

Amnesty said on Friday the searches "reinforce the menacing atmosphere for civil society" in Russia. The U.S. embassy in Moscow called them a cause for "great concern", and Europe's human rights commissioner urged an immediate halt to the raids.

Putin has accused foreign governments, including the United States, of meddling in Russia's domestic politics and Moscow expelled the U.S. Agency for International Development (USAID) last year saying it had tried to influence elections. (Reporting by Sonia Elks, editing by Gabriela Baczynska and Ruth Pitchford)

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